Shield for ignition plugs



Feb. 15, 1938. c. B. AIKEN ET AL SHIELD FOR IGNITION PLUGS Filed May 14, 1929 ,///////Vum wvmvrom AMEN 7T DURFE: J

' A TmR/VE'Y Patented Feb. 15, 1938 UNITED STATES.

PATENT-i OFFICE 2,108,835 SHIELD ion IGNITION rwcs Charles B. Aiken, New York, n.1, and Thomas Application may 14, 1929, Serial No. 362,940

2 Claims. (01. 123-189) The present: invention relates to reduction of interference in signaling'appalatus arising from the ignition system of a gas engine or similar source. More specifically the invention relates 5 to a means for suitably electrically shielding the ignition system, including the spark plug to reduce disturbances in adjacently located signaling apparatus suchas a radio transmitter or receiver. The invention has particular application to airplane motor ignition systems, but is applicable also to similar uses where practically complete shielding is to be obtained.

An object of the present inventionis to provide shielding which is adequate from the standpoint of protecting signaling apparatus from disturbances and which will stand up for indefinitely long periods .of time under any and all service conditions.

Shielding which'is adequate to suppress disturbances. inthe radio sets must extend to the entire ignition system including the spark plugs themselves. .to shield the ignition system from the magneto to the spark plug since this may be done by-com pletely covering the high tension circuit with a grounded'metal shield in the form of a metal tube or .a screen composed, for example, of braided copper wire.

The problem which has presented great difficulty in the past has been the provision of shielding for the spark plugs which would be adequate for reducing disturbances and at the same time would not in any way impair the operation of the motor even after long periods of use and under adverse weather conditions.

While it has been recognized for years that radio interference on an airplane could be reduced by shielding the ignition system attempts to apply such shielding have resulted in ignition failures in so'"many instances that apprehen 50"in wet weather with alternate heating andcooling of the motor.

The construction constituting the present invention will now be described indetail in connection with the attached drawing forming part 55 of this specification.

It is ,a comparatively'simple matter In the drawing Fig. 1 shows in simple, diagrammatic form the essentials of a typical ignition wiring system, Fig. 2 shows a part sectional view of a shielded cable and spark plug according to this invention, Fig. 3 is a similar view of a modi- 5 fled construction according to the invention, and Fig. 4 is a perspective view showing the construction of the clamping member of Fig. 3.

In Fig. 1 only so much of the ignition system isshown as is necessary to an undertsanding of 10 the shielding requirements. The high-tension magneto l supplies the requisite high voltage cur rent to the several ignition leads 2, 3, 4 etc., in succession, of which there may be any number depending on the type of motor. of the ignition circuits in common is formedby the ground connection 5 which may be the engine frame or chassis. Each lead 2, 3, 4 etc. extends to a motor cylinder, only one of which is indicated, at 6. The spark plug 1 'in each cylin- 20 der of the motor may be a commercial type of plug ofany suitable make or design. The lead 4 is shown as extending to the plug 1' and both the lead and the plug are covered over with a copper braid sheath which is kepth at ground poten- 5 tial by contact with the magneto casing and the cylinder of the motor as well as ground connections at several other points (not shown) along the sheath.

The leads 2, 3, 4 etc. each consists of a rubber- 30 covered cable having a single, usually stranded, copper conductor at the center. Between this conductor and the grounded sheath a high potential exists, which must be carried to and applied across the spark terminais within the motor 5 cylinder. To accomplish this successfully, the insulation resistance between the copper cable and the grounded sheath should be maintained at all times at a high value.

A description of the shielded plug construc- 0 tion of the invention will now be'given in connection with Fig. 2. The plug itself is shown as comprising a threaded base Ill, an apron ll, hexagonal assembly nut i2, insulator l3, and terminal it. 'As stated, the plug itself "may be a commercial type of any suitable design. v-

A hollow insulating cylinder l5, open at the lower end, is placed over the terminal l4 and extends well down over the insulator I3. The lower edge of the cylinder l5 may be beveled, as shown. This cylinder is preferably made ,of a ceramic product similar to porcelain, known as isolantite", a commercial insulating product of the lsolantite Company of America, located at Belleville, N. J. This material is preferred because One side of all 15,

' strength and the fact that it can be easily ma 2 or its high insulation resistance, mechanical chined before firing. Any other suitable insulation such as porcelain, a phenolic condensation product such as bakellte, or the like may be used,

however.

The cylinder l5'has an aperture atthe top to admit the rubber-covered cable l6 of the shielded connecting cable 4. The rubber insulation is shown in section at I! and the copper core at l8.

The latter is secured to the spring clip I9 as by passing through an aperture in the clip and being soldered to the metal of the clip. The clip engages the terminal H or the plug and makes good electrical contact therewith. The internal diameter of cylinder I5 is such that clip I! cannot be spread far enough to disengage terminal- I 4 when the cylinder i5 is once in place.

The copper braid covering 2| over the cable- 4 is spread as it approaches the plug and is drawn over the cylinder I5 and brought down-into contact with the base of the plug. It is there secured in place. in any suitable manner as by pass ing a few turns of wire!!! around it. A clamp 4 holds theshield 2| securely around the cable It at the point where the shield begins to spread.

To assemble the plug and shield, the braided covering 2| is drawn back over the cable l8 which is then cut to the proper length and secured to spring clip l9 after first being passed through" the aperture in the insulator IS. The latter is pushed upward on the cable l6 far enough to enable clip l9'to be placed on terminal 14. Then cylinder I5 is dropped over the plug until its lower edge engages the plug. The braided shielding is nowbrought over the whole and secured'in the position shown, by wire 20. 7

An important effect present in applicants structure is that the top of the struc'ure where the high tension lead is brought in remains cool due to the heat radiation from the copper braid.

Another important point is that the lead It passes through an insulator (l5) instead 01 through a grounded metal shield. A contributing factor also is the good contactbetween the which attacks the rubber. This is overcome in applicants device by the soldered connection and spring clip. The use of the s ring clip is facilitated by the cooling, the spring temper being retained indefinitely.

Too great cooling is not advantageous; The, sparking terminals and the lower end of the plug generally must be hot in order to prevent accumulation of soot or carbon which would partiallyshunt the gap. The construction must be such as to keep the sparking end hot and the opposite or top end sufllciently cool for reasons above given. In applicants structure, the top portion through which the cable passes never becomes too hot to be grasped by the bare .hand,

even after hours of flying.

The spreading of the shielding braid to pass over cylinder I515 advantageous in two ways. It

romotes cooling. am he greatenspacing from base or the plug by wrapping wire around the in Fig. 4.

by the spacing between any exposed high tension 5 part (I!) and the shield. Protection against conduction along the'path 34 may be further guarded against by filling the space between cable l6 and aperture wall in insulator II with rubber cement. These facts; particularly the fact that no grounded metal close to the outer surface of the rubber insulation becomes hot, make for long life and reliable service.

Instead of securing the braided. shield' to the shield as shown in Fig.1 a clamp of any suitable design such as a hose" clamp may be employed.

A special design of combined clamp and shield member is illustrated in Figs. 3 and 4.

The construction shown in Fig. 3 may be identical-with that in Fig. i so far as concerns the spark plug III, II, l2, l2 and M, the isolantite cylinderl5 and the disposition of the high tension cable I6 and spring clip l9.

After the cable I, clip I! and cylinder Ii have been placed in position as shown in Fig. 3, however, the cylinder is engaged and held firmly in place by the split sleeve 24 which is provided with screw clamping means indicated at 25 and 26 for tightening down the sleeve on the cylinder II and 30 the hexagonal portiongihe spark plug base. In

assembling, the split ve 24 will have been placed loosely in position before the clip I 8 and cylinder ii are moved into place. After these parts'have been positioned, the sleeve is made to grasp firmly the cylinder II" and the plug base by tightening down on screws 25 and 26. The construction of the split sleeve is shown more fully Aiter these parts. have been secured in place the braided sheath 2| is drawn down over the outside of the sleeve 24 and is served with wire 29 to holdthe braiding in place. To facilitate this a bulge is made at I. in the sleeve 24. Instead of using a sheathing or wire 29 ronthis P p se any suitable clamping means such as a split ring may be used. The sleeve 24 is made approximately hexagonal at the base in order to conform to the plug base and alsoprojects under the edge 01' the hexagonal base portion. -Instead ofbrlngipg thebralded sheath over the outside or the sleeve 24, it may be placed inside between sleeve 24 and insulator II and securely clamped in place by tightening screws 2' and 26.

It will be understood that the construction may be varied widely within the scope of the appended claims without. departing from the spirit of the invention, the forms above described being illustrative rather than limiting.

'Whatls claimed is:

1. In a shield construction for a spark plug, a ceramic housing over the P us terminal and surroimding the upper portion 01' said plug insulator having an aperture in its wall for admitting the high-tension lead wire, means inside the housihg tor'electrlcally cunnectinglthe lead wire to the plug terminal, and a metal shield extending over theoutslde oi the lead wire and the housing and contacting with the base of the plug. said ceramic housing serving to hold the metal shield away from the plug terminal and from the high ten-. sion lead wire and its rubber insulation where the latter-approaches and enters the said aperture.

2. A shield construction for a spark plug comprising a hollow insulator surrounding the upper portion of the plug insulator and enclosing the plug terminal and having an aperture through which the high'tension lead extends, and an expansible spring clip carried by the end of said high tension lead adapted to engage said plug terminal, the internal diameter of said hollow in- CHARLES B. AIKEN. THOMAS DURFEE. 

